“There are a lot of similar ultracompact dwarf galaxies, and together they may contain as many supermassive black holes as there are at the centers of normal galaxies.”

Seth and his fellow astronomers compiled data from the Hubble space Telescope along with the Gemini North 8-meter optical and infrared telescope in Hawaii to observe the dwarf galaxy and gauge the black hole’s mass. The Hubble took measurements of the galaxy’s diameter and stellar density while Gemini watched for motions affected by the enormous black hole.

How does M60-UCD1’s black hole compare to the Milky Way’s? The black hole at the center of our galaxy has the mass of around 4 million suns. That’s 0.01% of our galaxy’s total mass. M60-UCD1’s black hole on the other hand, has the mass of 21 million suns. That equals a whopping 15% of the tiny galaxy’s mass.

“That is pretty amazing, given that the Milky Way is 500 times larger and more than 1,000 times heavier than the dwarf galaxy M60-UCD1,” Seth said.

NASA touched on one potential explanation in its press release.

One explanation is that M60-UCD1 was once a large galaxy containing 10 billion stars, but then it passed very close to the center of an even larger galaxy, M60, and in that process all the stars and dark matter in the outer part of the galaxy were torn away and became part of M60.

The University of Utah put together a video showing how this would have happened.